1. Field of the Invention
The present invention relates to a defect inspection apparatus which inspects a defect of a mask.
2. Description of the Related Art
With an increase in integration density of a semiconductor device such as an LSI, a mask pattern that is formed on a mask, such as a reticle, has become finer. Accordingly, a defect inspection apparatus for a mask pattern is required to have higher performance. There has been proposed a defect inspection apparatus which detects a defect by acquiring an image of a mask pattern by means of an imaging device such as a CCD or a line sensor, and comparing the acquired image with a reference image (see, e.g. JP-A 10-177246 (KOKAI)).
Normally, when an image of a mask pattern is acquired, both transmission illumination and reflection illumination are used. The transmission illumination and reflection illumination have merits and demerits. By using both the transmission illumination and reflection illumination, their characteristics are made complementary.
However, if the positions of a transmission illumination region and a reflection illumination region are fixed, a desired image, in some cases, cannot be obtained. For example, an optical system has aberration and distortion. Although an image with high precision is obtained at a center of a visual field of an objective lens, but an image with high precision is hardly obtained at positions away from the center of the visual field of the objective lens. Consequently, if the transmission illumination region is positioned at the center of the visual field of the objective lens, the precision of a transmission illumination image is enhanced but the precision of a reflection illumination image is degraded. Conversely, if the reflection illumination region is positioned at the center of the visual field of the objective lens, the precision of a reflection illumination image is enhanced but the precision of a transmission illumination image is degraded.
As described above, if the positions of the transmission illumination region and reflection illumination region are fixed, such a problem arises that a desired image can hardly be acquired with high precision and a defect inspection cannot be performed with high precision.